Steel sheet for hot stamping use, method of production of same, and method of production of high strength part

ABSTRACT

The present invention has as its object the provision of steel sheet for hot stamping use which is excellent in part strength after hot stamping and delayed fracture resistance comprised of large C content high strength steel sheet in which effective hydrogen traps are formed in the steel material. The steel sheet of the present invention solves this problem by forming Fe—Mn-based composite oxides in the steel sheet and trapping hydrogen at the interfaces of the composite oxides and matrix steel and in the voids around the composite oxides. Specifically, it provides steel sheet for hot stamping use which is comprised of chemical ingredients which contain, by mass %, C: 0.05 to 0.40%, Si: 0.02% or less, Mn: 0.1 to 3%, S: 0.02% or less, P: 0.03% or less, Al: 0.005% or less, Ti: 0.01% or less, N: 0.01% or less, one or both of Cr and Mo in a total of 0.005 to 1%, and O: 0.003 to 0.03% and which have a balance of Fe and unavoidable impurities and which contains average diameter 0.1 to 15 μm Fe—Mn-based composite oxide particles dispersed in the steel sheet or furthermore has crushed voids around the composite oxide particles, a method of production of the same, and a method of production of a hot stamped high strength part.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of copending application Ser. No.14/003,881, filed on Sep. 9, 2013, which was filed as a National Phaseof PCT International Application No. PCT/JP2011/056124 on Mar. 9, 2011,all of which are hereby expressly incorporated by reference into thepresent application.

TECHNICAL FIELD

The present invention to steel sheet for hot stamping use which isexcellent in delayed fracture resistance, a method of production of thesame, and a high strength part which is formed by hot stamping usingthis steel sheet. In particular, it relates to a method of production ofa high strength part which is used for a structural part of anautomobile.

BACKGROUND ART

In recent years, reduction of the weight of automobiles has beenstrongly demanded from the viewpoint of the global environment. Inautomobile bodies, for example, pillars, door impact beams, bumperbeams, and other structural parts for automobiles, high strength steelsheet is being used to reduce the thickness of steel sheet to try tolighten the weight. For this reason, the strength of steel sheet isbeing raised. In particular, high strength steel sheet with a tensilestrength (TS) over 1000 MPa is being developed, but higher strength ofsteel sheet leads to a drop in the workability and press formability atthe time of production of a part. In particular, it becomes moredifficult to ensure product precision due to springback etc.

To solve these problems, in recent years, as a technique forsimultaneously satisfying higher strength and workability of the steelsheet and product precision, the hot stamping method (press quenchingmethod) has come to be used as a practical method. For example, this isdisclosed in PLT 1. This heats steel sheet to an approximately 900° C.or so austenite region, then press forms it hot and, at the time ofpress forming, brings it into contact with an ordinary temperature dieset to quench it and thereby obtain a high strength material. Due tothis hot stamping method, the residual stress which is introduced at thetime of press forming is also reduced, so the inconveniences offracture, poor shape freezing, etc. which become problems in highstrength steel sheet with a TS of over 1180 MPa are suppressed andproduction of parts with relative good product precision becomespossible.

In the high strength steel sheet which is used for automobiles etc., theabove-mentioned problems become more serious the higher the strength.Further, in particular, in high strength materials of over 1000 MPa, ashas been known in the past, there is the inherent problem of hydrogenembrittlement (also called “season cracking” or “delayed fracture”). Inthe case of steel sheet for hot pressing use, while the residual stressdue to pressing at a high temperature is small, hydrogen penetrates thesteel at the time of heating before pressing and the susceptibility tohydrogen embrittlement becomes higher due to the residual stress afterpressing.

As the method of preventing cracking due to delayed fracture, there isthe method of controlling the heating atmosphere at the time of hotstamping. For example, PLT 2 proposes the method of making the hydrogenconcentration in the heating atmosphere of the hot stamping 6 vol % orless and making the dew-point 10° C. This relates to a method of controlof the heating atmosphere of hot stamping. That is, by controlling thehydrogen concentration and the condensation point, the penetration ofexternal hydrogen into the steel sheet during heating is suppressed.Therefore, this does not improve the steel sheet itself. It can only beapplied in hot stamping which has a system for controlling theatmosphere.

In addition, as the steel sheet for hot stamping use, there is knownsteel sheet which traps the hydrogen which penetrates the steel sheetand thereby prevents delayed fracture. For example, PLT 3 proposes steelsheet for hot stamping use which improves the delayed fractureresistance. This art incorporates average particle size 0.01 to 5.0 μmrange Mg oxides, sulfides, composite crystals, and compositeprecipitates, e.g. one or more composite oxides among them, into thesteel in an amount of 1×10² to 1×10⁷ per square mm. These oxides andcomposite crystals and composite precipitates having these as nuclei actas hydrogen trap sites to thereby improve the delayed fractureresistance.

Further, as similar art, PLT 4 discloses the art of producing highstrength thin-gauge steel sheet which is excellent in hydrogenembrittlement resistance characterized by making bainite or martensitethe biggest phases in terms of area rate, making one or more of Nb, V,Cr, Ti, and Mo oxides, sulfides, nitrides, composite crystals, andcomposite precipitates in the particles satisfy an average particle size“d”: 0.001 to 5.0 μm, a density p: 100 to 1×10¹³/mm², and a ratio ofstandard deviation σ of average particle size and average particle size“d”: σ/d≤1.0, and by having a tensile strength of 980 MPa or more.

Furthermore, in steel sheet for enameling use, to improve the fishscalesusceptibility, it is known that it is effective to form voids in thesteel sheet to trap the hydrogen. PLT 5 proposes to form Fe—Nb—Mn-basedcomposite oxides in steel sheet and increase the segregation of Nb andMn in the oxides so as to raise the hydrogen trapping ability. However,the art which is described in PLT 5 is art which assumes steel sheet forenameling use which has a small C (carbon) content (usually 0.01 mass %or less). In large C content high strength steel sheet (C of 0.05 mass %or more) such as steel sheet for automobile use, the oxidizing action ofC cannot be ignored. Therefore, this cannot be simply applied.

Further, the amount of hydrogen problematic in steel sheet for enamelinguse is a high concentration of 10 to 100 ppm, while with high strengthsteel sheet, an amount of hydrogen of a very low concentration of 1 to 3ppm is considered a problem.

Therefore, the art which is described in PLT 5 cannot be applied as isto large C content high strength steel sheet.

To apply these arts to large C (carbon) content high strength steelmaterials, suitable control of the size (average particle size) andpresence (density) of the oxides etc. present in the steel sheet is animportant requirement. However, strict control to give a particle sizeand density which are effective as hydrogen trap sites and which do notform starting points of coarse cracks is not technically easy.

CITATIONS LIST Patent Literature

-   PLT 1: Japanese Patent Publication No. 10-96031A-   PLT 2: Japanese Patent Publication No. 2006-51543A-   PLT 3: Japanese Patent Publication No. 2006-9116A-   PLT 4: Japanese Patent Publication No. 2005-68548A-   PLT 5: WO2008/038474A

SUMMARY OF INVENTION Technical Problem

Above, the state of the art regarding measures against delayed fracturedue to hydrogen embrittlement of hot stamped steel sheet was explained.The problem is that there is at the present point of time no art whichsuppresses delayed fracture due to hydrogen embrittlement when hotstamping large C content high strength steel sheet.

Therefore, an object of the present invention is to provide steel sheetfor hot stamping use which is excellent in part strength after hotstamping and delayed fracture resistance comprised of large C contenthigh strength steel sheet in which strength is secured while effectivehydrogen traps are formed in the steel material, a method of productionof the same, and a method of production of a hot stamped high strengthpart.

Solution to Problem

The inventors took note of the fact that, to improve steel sheet for hotstamping use in delayed fracture resistance, trapping the hydrogen whichpenetrates the steel sheet is effective and engaged in intensiveresearch based on that. As a result, they discovered that it is possibleto cause the formation of Fe—Mn-based composite oxides in steel sheetand trap the hydrogen at the interfaces of the composite oxides and thematrix steel and thereby completed the present invention.

In large C content high strength steel sheet, usually the inclusions ofmetal oxides become defects. For this reason, as much as possible, theoxygen in the steel is removed and formation of metal oxides issuppressed. Therefore, in addition to adding Al and other deoxidizingelements, the concentration of oxygen is reduced at the stage of moltensteel.

However, to cause the formation of Fe—Mn-based composite oxides in thesteel like in the present invention, it is necessary to leave oxygen inthe steel to a certain extent. Further, C itself has a deoxidizingaction, so in general, with large C content steel sheet, the oxygen inthe steel ends up becoming small in amount.

Therefore, the inventors discovered that by reducing the concentrationof Al in the steel sheet, weakening the deoxidizing effect, and securinga concentration of oxygen in the steel, it is possible to cause theformation of composite oxides even in large C content steel sheet.

Further, they discovered that to raise the hydrogen trapping effect ofcomposite oxides, it is effective to crush the composite oxides andincrease their surface area. They discovered that by crushing and makingthe composite oxides finer, their effect as defects falls and this leadsto an improvement in the performance of the steel sheet.

Furthermore, they learned that if there are voids around the compositeoxides, the hydrogen trapping effect is improved more.

The inventors engaged in intensive studies on the method of productionfor the above.

They learned that large C content molten steel is high in viscosity, soFe—Mn-based composite oxides have difficulty rising and steelFe—Mn-based composite oxides can be easily formed in the steel.

Further, it was learned that by rolling (hot rolling or further coldrolling) a slab comprised of steel in which Fe—Mn composite oxides areformed, the composite oxides can be stretched and crushed. In this way,they discovered that it is possible to efficiently form hydrogen trapsites in steel sheet which do not become starting points of cracks.Further, they discovered that it is possible to form effective voids ina similar process. The present invention was completed based on thesediscoveries. The present invention has as its gist the following:

(1) Steel sheet for hot stamping use which is comprised of chemicalingredients which contain, by mass %,

C: 0.05 to 0.40%, Si: 0.001 to 0.02%, Mn: 0.1 to 3%, Al: 0.0002 to0.005%, Ti: 0.0005 to 0.01%, O: 0.003 to 0.03%,

one or more of Cr and Mo in a total of 0.005 to 2%, anda balance of Fe and unavoidable impurities,wherein the steel sheet contains average diameter 0.1 to 15 μmFe—Mn-based composite oxide particles dispersed in the steel sheet.Note that, S, P, and N are unavoidable impurities, but are restricted tothe following contents:S: 0.02% or less,P: 0.03% or less,N: 0.01% or less,(2) The steel sheet for hot stamping use as set forth in (1) whichfurther contains, by mass %, the ingredients which are included in oneor more groups among the three groups of (a) to (c):

(a) B: 0.0005 to 0.01%;

(b) one or more of Nb, V, W, and Co in a total of 0.005 to 1%; and(c) one or more of Ni and Cu in a total of 0.005 to 2%.(3) The steel sheet for hot stamping use as set forth in (1) or (2),wherein there are voids around the composite oxide particles.(4) The steel sheet for hot stamping use as set forth in (1) or (2),wherein the voids around the composite oxide particles have averagesizes of 10 to 100% of the average size of the composite oxideparticles.(5) The steel sheet for hot stamping use as set forth in (1) or (2),wherein the steel sheet is plated by any of aluminum plating,zinc-aluminum plating, and zinc plating.(6) A method of production of steel sheet for hot stamping usecomprising hot rolling a slab of chemical ingredients set forth in (1)or (2) in which rough rolling the slab by a rolling rate of 70% or moreand final rolling the slab by a rolling rate of 70% or more.(7) The method of production of steel sheet for hot stamping use as setforth in (6), further comprising pickling the hot rolled steel sheetwhich was obtained by hot rolling and cold rolling the steel sheet by arolling rate of 30% or more.(8) The method of production of steel sheet for hot stamping use as setforth in (7), further comprising annealing the cold rolled steel sheetwhich was obtained by cold rolling.(9) A method of production of a high strength part using the steel sheetfor hot stamping use comprising heating the steel sheet as set forth in(1) or (2) to a temperature of austenite region of the Ac₃ or higher,then starting to form the steel sheet by a die, and cooling the steelsheet in the die after forming to quench.

Advantageous Effects of Invention

The high strength steel sheet for hot stamping use of the presentinvention stretches and crushes composite oxides to thereby formcomposite oxide particles and their surrounding voids which areeffective as hydrogen trap sites. Due to this, there is no need tostrictly control the size (average particle size) and state of presence(density) of oxides etc. like in the past and it is possible to providesteel sheet which is excellent in delayed fracture characteristics. Ifusing a member which is produced from the steel sheet of the presentinvention, it is considered possible to greatly contribute to thelighter weight and greater safety of automobiles. The contribution toindustry is great.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view which shows the state where coarse compositeoxides are stretched and crushed and many crushed voids (hydrogentrapping ability) are formed in the steel sheet.

FIG. 2 is a schematic view which shows the state where coarse oxides arestretched and crushed and few crushed voids (hydrogen trapping ability)are formed in the steel sheet.

FIG. 3 is a schematic view which shows that crushed voids are not formedwhen there are fine oxides present.

FIG. 4 is a cross-sectional view of the shape of a die set which is usedin the examples.

FIG. 5 is a view which shows the shape of a punch which is used in theexamples as seen from the top.

FIG. 6 is a view which shows the shape of a die which is used in theexamples as seen from the bottom.

FIG. 7 is a schematic view of a hot stamped part.

FIG. 8 is a view which shows the shape of a test part for evaluation ofdelayed fracture resistance as seen from the top.

DESCRIPTION OF EMBODIMENTS

Below, the present invention will be explained in detail.

The fact that delayed fracture occurs due to the diffusible hydrogenwhich penetrates the steel sheet from the outside environment anddiffuses in the steel sheet at room temperature is already known.Therefore, if able to trap hydrogen which penetrates from the outsideenvironment at some part inside the steel sheet, it would becomepossible to render the hydrogen harmless and delayed fracture would besuppressed.

The inventors discovered that by casting a slab comprised of steel inwhich Fe—Mn-based composite oxides are formed in the steelmaking processand by hot rolling and cold rolling the slab to stretch and crush thecomposite oxides, it is possible to form fine voids between the finelycrushed Fe—Mn-based composite oxide particles, that the voids areeffective as hydrogen trap sites, that diffusible hydrogen, which isbelieved to be the cause of delayed fracture, is trapped at those parts,and the susceptibility to delayed fracture falls. Furthermore, theinventors discovered that these voids were of sizes and shapes by whichthey did not easily become starting points of cracks and attempted toapply the steel for a hot stamping material in which strength isdemanded.

First, the reasons for limiting the strength of a part after hotstamping of the present invention and the ingredients of steel sheet forhot stamping use which is excellent in delayed fracture resistance topredetermined ranges will be explained. Here, for the ingredients, %means mass %.

C: 0.05 to 0.40%

C is an element which is added to make the structure after coolingmartensite and secure the material quality. To improve the strength,0.05% or more of C is necessary, but if the C content exceeds 0.40%, thestrength at the time of deformation upon impact and the weldabilitydeteriorate, so C was made 0.05 to 0.40%. From the viewpoint of thestrength, furthermore, the C content is preferably made 0.15% or more,more preferably is made 0.2% or more.

Further, from the viewpoint of the deterioration of the strength at thetime of deformation upon impact or the weldability and the effect ofdeoxidation by C, the C content is preferably 0.35% or less, morepreferably 0.3% or less.

Si: 0.001 to 0.02%

Si acts as a deoxidizing element. The present invention requires that acertain amount or more of oxides be secured, so Si, which reduces theoxygen content, is limited to 0.02% or less. To obtain the amount ofeffective oxides, the Si content is made 0.015% or less, more preferably0.01% or less. The lower limit of the Si content is not particularly anissue, but due to the time and expense involved in removing Si, 0.001%is made the lower limit.

Mn: 0.1 to 3%

Mn is an element which affects the hot stampability and hardenabilityand is effective for raising the strength of the steel sheet. Further,Mn, by addition, forms Fe—Mn composite oxides, so is an importantingredient in the present invention. These composite oxides form trapsites for the hydrogen which causes delayed fracture. For this reason,addition of Mn is effective for improvement of the delayed fractureresistance.

Further, the formed composite oxides are fine in size, so are effectivefor suppressing the formation of coarse cracks at the punched surfaces.To form oxides and utilize Mn to the maximum extent as hydrogen trapsites, it is sufficient to proactively add Mn since addition facilitatescontrol of the oxide composition. If Mn is less than 0.1%, this effectcannot be obtained. For this reason, the Mn content may be made 0.1% ormore. To reliably obtain this effect, the Mn content is preferably made0.5% or more. Furthermore, 1.30% or more is more preferable.

Further, if the Mn content exceeds 3.0%, the Mn assists co-segregationwith P and S, invites a drop in the toughness, and lowers the delayedfracture resistance. For this reason, the Mn content should be made 3%or less. More preferably, the Mn content may be made 2.0% or less, morepreferably 1.50% or less.

S: 0.02% or Less

S is contained as an unavoidable impurity. If contained in excess, itdegrades the workability, becomes a cause of deterioration of toughness,and lowers the delayed fracture resistance. For this reason, the smallerthe S, the better. As the allowable range, the content is defined as0.02% or less. Preferably, the content should be made 0.01% or less.Furthermore, by limiting the S content to 0.005% or less, the impactcharacteristics are strikingly improved.

P: 0.03% or Less

P is an element which is contained as an unavoidable impurity and has adetrimental effect on toughness when excessively added. It lowers thedelayed fracture resistance. For this reason, the less the P, thebetter. As the allowable range, the content is limited to 0.03% or less.Furthermore, 0.025% or less is preferable. Furthermore, if 0.02% orless, the effect of improvement of the delayed fracture resistance islarge.

Al: 0.0002 to 0.005%

Al is an element which is required for use as a deoxidizing material ofmolten steel. The present invention requires that a certain amount ormore of oxides be secured, so if Al, who has a deoxidizing effect, isover 0.005%, the amount of oxides for improving the delayed fractureresistance cannot be secured. For this reason, the upper limit was made0.005%. If considering a margin of safety, the Al content is preferablymade 0.004% or less, more preferably is made 0.003% or less. Further,the lower limit is not particularly set, but removing Al involves timeand expense, so 0.0002% or more is practical.

Ti: 0.0005 to 0.01% or Less

Ti is also a deoxidizing element. The lower limit is not particularlyset, but removing Ti involves time and expense, so the content issufficiently made 0.0005% or more, preferably 0.001% or more. On theother hand, addition of a large amount reduces the oxides which improvethe delayed fracture resistance, so the upper limit was made 0.01%.Furthermore, 0.008% or less is preferable. Furthermore, if 0.006% orless, the effect of improvement of the delayed fracture resistance islarge.

N: 0.01% or Less

If N is over 0.01%, the nitrides coarsen and the dissolved N causeshardening upon ageing, whereby a tendency for the toughness todeteriorate is seen. For this reason, the smaller the N the better. Asthe allowable range of N, the content is limited to 0.01% or less inrange. Preferably, it is made 0.008% or less. If 0.006% or less, it ispossible to suppress the deterioration of toughness, so this ispreferable.

One or Both of Cr and Mo in Total of 0.005 to 2%

Cr and Mo are both elements which improve the hardenability. Further,they have the effect of causing precipitation of M₂₃C₆ type carbides inthe matrix and have the action of raising the strength and refining thecarbides. For this reason, one or both of Cr and Mo are added in a totalof 0.005 to 2%. If less than 0.005%, these effects cannot besufficiently expected. More preferably, the content should be made 0.01%or more. Furthermore, if 0.05% or more, the effect becomes remarkable.Further, if exceeding 2% in total, the yield strength excessively rises,the toughness is degraded, and the delayed fracture resistance islowered. If possible, from the viewpoint of the delayed fractureresistance, the content is more preferably made 1.5% or less.

(O: 0.003 to 0.03%)

O is an element which is required for forming Fe—Mn composite oxides inthe present invention. Inclusion of 0.003 to 0.03% is necessary. If lessthan 0.003%, a sufficient amount of Fe—Mn composite oxides cannot beformed. From the viewpoint of forming Fe—Mn composite oxides, 0.005% ormore is preferable. On the other hand, if including over 0.03%, the castslab ends up with blowholes and other internal defects, so the upperlimit was made 0.03%. From the viewpoint of internal defects, less isbetter. An O content of 0.02% or less is preferable. If possible, if0.015% or less, the defects remarkable decrease.

B: 0.0005 to 0.01%

B is an element which is effective for improving the hardenability. Tomake this effect more effective, addition of 0.0005% or more isnecessary. To make this effect more reliable, 0.001% or more ispreferable. Furthermore, 0.0015% or more is more preferable. On theother hand, even if excessively added, the effect becomes saturated, so0.01% was made the upper limit. Seen from the viewpoint of cost versuseffect, 0.008% or less is preferable. If possible, 0.005% or less ismore preferable.

One or More of Nb, V, W, and Co in Total of 0.005 to 1%

Nb, V, W, and Co are carbide-forming elements. They form precipitates tosecure the strength of the hot stamped and quenched member. Furthermore,these are necessary elements which are contained in the Fe—Mn-basedcomposite oxides, act as hydrogen trap sites which are effective forimprovement of the delayed fracture resistance, and improve the delayedfracture resistance. One or more of these elements may be added. If theamounts of addition exceed a total of over 1%, the rise in the yieldstrength excessively increases. For this reason, 0.7% or less is morepreferable. If possible, 0.5% or less is still more preferable. On theother hand, if less than 0.005%, the improvement in strength and theeffect as a hydrogen trap site become difficult to obtain. From theviewpoint of reliably obtaining this effect, 0.01% or more ispreferable.

One or Both of Ni and Cu in Total of 0.005 to 2%

Ni and Cu are both elements which improve the strength and toughness,but if added in a total of over 2%, the castability falls, so the upperlimit is made 2%. From the viewpoint of the castability, the content maybe reduced. 1% or less is more preferable. 0.5% or less is morepreferable. On the other hand, if less than 0.005% in total, the effectof improvement of the strength and toughness are difficult to obtain, soone or both of Ni and Cu may be added in a total of 0.005% or more. Fromthe viewpoint of the strength and toughness, 0.01% or more ispreferable. Furthermore, 0.02% or more is more preferable.

Next, the method of production of steel sheet for hot stamping use whichis excellent in delayed fracture resistance of the present inventionwill be explained.

In the present invention, it is possible to smelt steel adjusted incomposition of ingredients of the present invention by the usualsmelting, continuous casting, and steel sheet production process. Inparticular, to form the Fe—Mn-based composite oxides characterizing thepresent invention, it is preferable to add the weak deoxidizing abilityelements first in the steel smelting and casting processes. For example,by adding Mn, Si, Al, etc. in that order, the effect of the presentinvention can be obtained more remarkably.

The mechanism by which these steelmaking conditions affect theproperties of the invention steels is believed to be the following: Thefluctuations in composition of the composite oxides of the inventionsteels are mainly due to the fluctuations in composition of thethermodynamic oxides at the time of melting and solidifying the steels.Basically, this is realized by utilizing the nonequilibrium state in theprocess of the composition of oxides approaching the equilibrium statedue to the change in concentration and change in temperature of thesystem. By adding a weak oxidation ability element A first, the oxygenin the molten steel forms coarse oxides of A, but by adding an element Bwith a strong bonding force with oxygen after that, the element A in theoxides of A is switched to the element B. In the process, coarsecomposite oxides of A and B (A-B composite oxides) are formed. If endingup adding the strong deoxidation ability element first, formation of acomposite after that becomes difficult. Not only that, a large amount ofoxides are formed together with addition and deoxidation occurs. Thelarge amount of oxides float up in the molten steel make dispersion ofoxides into the steel difficult. As a result, the effect of improvementof the delayed fracture resistance of the product is reduced.

Due to such a mechanism, time is required for forming coarse compositeoxides after addition of a weak oxidizing element. On the other hand, ifan excessively long time ends up elapsing after addition of an element,the composition of the A-B composite oxides becomes too close to theoxides of B in the equilibrium state. Not only does the effect of thecomposite oxides become smaller, but also the oxides again float up andend up leaving the molten steel so the effect of improvement of thecharacteristics is inhibited.

The voids which function as hydrogen trap sites are mainly formed in thecold rolling process after hot rolling. That is, the Fe—Mn-basedcomposite oxides are crushed by the rolling whereby crushed voids areformed around the composite oxide particles. For this reason, it isimportant to control the shape of the composite oxides in the hotrolling process.

In the present invention, the composite oxide parties which aredispersed in the steel were originally an integrated composite oxide.That is, at the time of casting the molten steel finished being adjustedin ingredients, there was a single large oxide mass, but this isbelieved to be stretched, crushed, and finely dispersed in the rollingprocess. Such stretching and crushing mainly occurs in the rollingprocess. When the temperature of the steel sheet is high (1000° C. ormore), oxides are mainly stretched.

On the other hand, when the temperature of the steel sheet is low (1000°C. or less), the oxides are mainly crushed. In such a process, if thereis segregation in composition in the oxides, the extent of stretchingwill differ depending on the portion of the oxides and the shape of theoxides will become complicated. Further, the fine (thin) portions arepreferentially crushed, while the portions with large fluctuations inshape are expected to be preferentially crushed due to the concentrationof deformation stress. As a result, portions which differ in compositionare efficiently crushed and dispersed. At the time of this crushing,voids are sometimes formed around the composite oxide particles. Thesealso become hydrogen trap sites in the steel and are believed toremarkably improve the delayed fracture resistance of the hot stampedproducts.

The above will be explained with reference to the figures.

FIG. 1 is a schematic view which shows the state where coarse compositeoxides are stretched and crushed and a large number of crushed voids(hydrogen trapping ability) are formed in the steel sheet. In FIG. 1,the coarse composite oxides 1 are formed by two different types ofoxides 1-1 and 1-2 as composites. The composite oxides 1 are hot roughrolled 2 (shown by arrows in FIG. 1) to stretched composite oxides 3 andthe oxides 3-1 and 3-2 are also stretched. Next, they are hot finalrolled 4 (shown by arrows in FIG. 1) and further stretched and crushed.At this time, oxides of different hardnesses are crushed, so numerouscrushed voids 5 are formed around the particles 5-1 and 5-2 of thecrushed composite oxides. These crushed voids 5 also become hydrogentrap sites whereby the delayed fracture resistance is improved.

As opposed to this, the case where, like in the past, just coarse oxidesare contained is shown in FIG. 2. The coarse oxides 6 are hot roughrolled 2 (shown by arrows in FIG. 2) to become stretched oxides 7. Next,they are hot final rolled 4 (shown by arrows in FIG. 1) to be stretchedand crushed. However, since these are masses of oxides, the crushedoxides 8 also do not disperse as fine composite oxide particles such asin the present invention. Therefore, it is not possible to obtaincrushed voids 5 which are sufficient as hydrogen trap sites.

FIG. 3 is a schematic view which shows that crushed voids are not formedbefore hot rolling, that is, there are fine oxides at the slab stage. Iffine composite oxides 6′ at the slab stage such as in FIG. 3, the fineoxides 6′ are hard to stretch by rough rolling 2 (shown by arrows inFIG. 3). As a result, even with final rolling 4 (shown by arrows in FIG.3), the oxides are not crushed that much, so crushed voids 5 which formhydrogen trap sites become difficult to form.

Note that, while not shown, cold rolling, in the same way as hot finalrolling 4 (shown by arrows in FIGS. 1 to 3), has the effect of furtherfinely crushing the oxides.

To efficiently trap the hydrogen, it is desirable that the compositeoxide particles uniformly disperse in the steel sheet. Further, theinterfaces between the composite oxide particles and the matrix steelbecome hydrogen trap sites, so the composite oxide particles should havelarge specific surface areas (surface areas per unit weight). For thisreason, the composite oxides are desirably fine. Further, from theviewpoint of suppression of defects as well, the composite oxides aredesirably fine.

Furthermore, the voids which are formed around the composite oxideparticles also become smaller if the composite oxide particles aresmall. Therefore, from the viewpoint of reducing the volume of voids inthe steel sheet as well, the composite oxides preferably become finer.Further, the fact that rolling enables the composite oxides to bestretched, crushed, and made finer is convenient since this is possiblewith current processes as they are.

The Fe—Mn-based composite oxides which are covered by the presentinvention are Fe—Mn-based composite oxides comprised of oxides of Fe,Mn, Si, Al, etc. joined together as composites. The composite oxides arepreferably fine in size, but if too fine in size, the hydrogen trappingeffect is reduced. Therefore, the diameter of the composite oxides ispreferably 0.10 μm or more. This is because in oxides which are smallerthan this range, the great feature in the characteristics of the steelsheet of the present invention, that is, the effect as hydrogen trapsites becomes extremely small. Preferably, it is 0.50 μm or more, morepreferably 1.0 μm or more, still more preferably 2.0 μm or more.

The upper limit of the diameter does not have to be particularly limitedif considering the effect of the present invention. However, whiledepending on the contained oxygen, if the coarse composite oxides becomegreater, the number density of the composite oxides will decrease andthe hydrogen trapping effect will become smaller. Further, too coarseoxides, as is generally known, become starting points of cracking of thesteel sheet when working the product sheet and thereby impair theworkability. If considering these, the average diameter of the compositeoxides is preferably kept to 15 μm or less, preferably 10 μm or less,more preferably 5 μm or less.

The average diameter of the oxides and the voids near the oxides arepreferably observed by an optical microscope or scan type electronmicroscope after polishing a cross-section of the steel sheet.Furthermore, for detailed observation, the steel sheet is preferablyused to prepare a thin film sample, then is observed by a transmissiontype electron microscope. Measurement of the voids is, for example,described in JIS (Japanese Industrial Standard) G0555 “Microscopic TestMethods of Nonmetallic Inclusions of Steel”.

Similarly, when crushed voids are formed, their sizes are notparticularly limited. The size of a void is a long axis of 0.1 to 5 μmfor an aspect ratio of 2 to 10. However, if the crushed voids are toolarge, void defects result and the characteristics of the steelmaterials are degraded. Usually, the size is the size of the crushedcomposite oxides. Therefore, the average size of the crushed voidsbecomes 100% or less of the average size of the composite oxides(particles). From the viewpoint of the defects, the voids should also besmall. Preferably, they should be 80% or less. The lower limit of theaverage size of voids is not particularly set. Even if the average sizeis 0, that is, there is no void, hydrogen trap sites are formed by theinterfaces of the composite oxides and steel.

The “average size of the voids” in the present invention is defined asthe average value of the long axes and short axes of five voids.

Hot rolling, in particular rough rolling is high in temperature, so thecomposite oxides also soften and the difference in hardness from thematrix iron is also small. That is, in the temperature region of roughrolling, that is, about 1000° C. or more temperature region, there isalmost no fracture of composite oxides due to rolling and the compositeoxides are stretched.

Further, if lower than 1000° C., preferably 900° C. or less, thecomposite oxides become difficult to stretch. At the prior stage of hotfinal rolling, fracture of the extent where fine cracks are formed occurat part of the composite oxides. Furthermore, at the final stage of hotrolling or at cold rolling, the composite oxides are crushed startingfrom the fine cracks which were formed. To obtain composite oxides whichare suitably stretched and simultaneously have fine cracks and arecrushed in this way, temperature control at the time of hot rolling andcontrol of the strain and strain rate at different temperature regionsbecomes necessary.

If the temperature region of the hot working is too high, it is notpossible to impart enough strain for forming cracks to the compositeoxides. Further, if too low, the composite oxides are not stretched instate, but become close to spherical shapes, so cracks are difficult toform. Suitable stretching and reduction of thickness is necessary forformation of cracks. For this reason, it is necessary to control andimpart stretching of the composite oxides by suitable deformation at ahigher temperature in hot rolling and formation of cracks in the lowtemperature region. Further, the form of the composite oxides which formsuch cracks, as explained above, becomes more complex when there is adifference in concentration inside the composite oxides and a differencein deformation ability. Efficient formation of effective voids becomespossible.

The hot rolling heating temperature and coiling temperature etc. of thehot rolling conditions can be set as usual in the usual operatingregion. To sufficiently obtain the effect of stretching the compositeoxides in hot rolling, the hot rolling heating temperature should bemade 1000 to 1400° C. Preferably, it should be made 1050° C. or more.Due to this, hot rough rolling can be performed at 1000° C. or more and,after that, hot final rolling can be performed at 1000° C. or less. Thelast final rolling temperature should be made 800° C. or less.Preferably, it should be made 750° C. or less. Due to this, thestretched composite oxides are increasingly crushed. Making the coilingtemperature 700° C. or less is advantageous economically.

Further, to control the form of the composite oxides, the sheet ispreferably rough rolled by a rolling rate of 70% or more and finalrolled by a rolling rate of 70% or more. The higher the rolling rate,the more effective in crushing and stretching the composite oxides, sothe sheet is more preferably rough rolled by a rolling rate of 75% ormore. 80% or more is more preferable. Further, it is still morepreferable if the rolling rate in final rolling is 80% or more. 90% ormore is more preferable. That is, with this rolling rate, the compositeoxides are stretched and crushed and become hydrogen trap sites whichare effective for improvement of the delayed fracture resistance.

In hot rolling as well, composite oxide particles which become hydrogentrap sites are obtained, but further cold rolling enables the compositeoxides to be made finer and thereby the hydrogen trapping effect to beimproved. For the cold rolling to sufficiently crush the compositeoxides, the rolling rate in the cold rolling should be made 30% or more.This is because with a 30% or more cold rolling rate, the compositeoxides are stretched and crushed to form hydrogen trap sites which areeffective for improving the delayed fracture resistance and the delayedfracture resistance is further improved. Furthermore, 40% or more ismore preferable, while if 50% or more, the improvement in the delayedfracture resistance becomes remarkable. In particular, when deep drawingbecomes necessary, it is preferable to make the rolling rate in coldrolling 60% or more.

In the case of annealing, either the continuous annealing method or thebox annealing method which is performed on ordinary cold rolled steelsheet may be used.

When the steel sheet for hot stamping use is used as a structural partfor an automobile, it is mostly used treated on its surface. Inparticular, it is mostly used as plated steel sheet. As plated steelsheet, usually aluminum plated, zinc-aluminum plated, and zinc platedsheet are used. The steel sheet for hot stamping use of the presentinvention may also be plated by ordinary methods. For example, whenapplying hot dip aluminum coating, the surface of the steel sheet shouldbe coated by 30 to 100 g/m² or so at one side.

Further, to produce a high strength part by hot stamping in the presentinvention, the steel sheet is first heated in the austenite region, thatis, to the Ac₃ transformation point or higher austenite region. In thiscase, it is sufficient that the austenite region be reached. If toohigh, coarsening of the particles or oxidation will become remarkable,so this is not preferred. Next, the sheet starts to be shaped by the dieset. By constraining the part after being worked by the die set whilerapidly cooling it and causing martensite transformation for quenching,it is possible to produce a high strength part.

If the cooling rate becomes slow, quenching is no longer achieved andthe target strength can no longer be obtained, so the speed of rapidcooling from the austenite region is made the critical cooling ratewhich is affected by the steel ingredients or structure or more. Thecooling completion temperature is preferably the martensitetransformation completion temperature or less.

Note that, tempering need not particularly be performed, but may beperformed in accordance with need for correcting too high strength orimproving the toughness.

EXAMPLES

Below, examples will be used to explain the present invention.

Example 1

Steels of the chemical ingredients which are shown in Tables 1-1 to 1-3and Tables 2-1 to 2-3 were cast to produce slabs. Note that, Tables 2-1to 2-3 show steel types which have the Steel Types A, X, and AC whichare described in Table 1-1 and Table 1-2 as base steels and havedifferent ingredient elements which are described in Tables 2-1 to 2-3mixed in with them.

These slabs were heated to 1050 to 1350° C. and hot rolled at a finishtemperature 800 to 900° C. and a coiling temperature 450 to 680° C. toobtain thickness 4 mm hot rolled steel sheets. After that, the sheetswere pickled, then were cold rolled to obtain thickness 1.6 mm coldrolled steel sheet. After that, they were continuously annealing(annealing temperature 720 to 830° C.). Further, parts of the coldrolled steel sheets were hot dip galvanized (basis weight: one side 30to 90 g/m²), hot dip galvannealed (basis weight: one side 30 to 90g/m²), and hot dip aluminum coated (basis weight: one side 30 to 100g/m²) on a continuous hot dipping line. The steel sheet types are shownin Tables 1-1 to 3 and 2-1 to 3. The types of steel sheets are shownbelow:

HR: hot rolled steel sheet, CR: cold rolled steel sheet (annealedmaterial), AL: hot dip aluminum coated steel sheet, GI: hot dipgalvanized steel sheet, and GA: hot dip galvannealed steel sheet.

The average (arithmetic average) particle size of the Fe—Mn compositeoxides in a produced steel sheet and the presence of crushed voids weredetermined by polishing a cross-section of the steel sheet, thenobserving it by an optical microscope or scan type electron microscopeor by a transmission type electron microscope after preparing the sampleinto a thin film. The results are shown together in Tables 1-1 to 3 andTables 2-1 to 3. The judgment criteria are shown below:

Average particle size of composite oxides:Good: average diameter 0.1 to 15 μm,Poor: average diameter less than 0.1 μm or over 15 μmAn average diameter of the composite oxides, as explained above, of 0.1to 15 μm was deemed as passing.Crush voids around composite oxides:Good: average size of voids 0.1 μm or more,Poor: average size of voids less than 0.1 μm.The average size of the crushed voids around the composite oxides, asexplained above, is preferably 0.1 μm or more.

After that, these cold rolled steel sheets were heated by a heatingfurnace to over the Ac3 point, that is, the 880 to 950° C. austeniteregion, then were hot worked. For the atmosphere of the heating furnace,combustion exhaust gas was used. The hydrogen concentration in theatmosphere was 2%, while the dew-point was 20° C.

A cross-section of the die set shape is shown in FIG. 4. FIG. 4 showsthe shapes of a die 9 and punch 10. The shape of the punch when seenfrom above is shown in FIG. 5. FIG. 5 shows the punch 10. The shape ofthe die when seen from below is shown in FIG. 6. FIG. 6 shows the die 9.In the die set, the shape of the die is determined based on the punchwith a clearance of the sheet thickness of 1.6 mm. The blank size wasmade 1.6 mm thickness×300 mm×500 mm. The shaping conditions were made apunch speed of 10 mm/s, a pressing force of 200 tons, and a holding timeat bottom dead center of 5 seconds. A schematic view of the hot stampedpart 11 is shown in FIG. 7.

The quenching characteristic of the hot stamped part was evaluated bypolishing the cross-section, corroding it by Nital, then observing themicrostructure by an optical microscope and determining the area rate ofmartensite. The results of judgment are shown in Tables 1-1 to 1-3 andTables 2-1 to 2-3. The judgment criteria are shown below:

Good: martensite area rate 90% or more,Fair: martensite area rate 80% or more, andPoor: martensite area rate less than 80%.A martensite area rate of 80% or more was deemed the preferable range.

The delayed fracture resistance was evaluated by imparting stress bypiercing. The pierce hole position 13 at the center of the test part 12which is shown in FIG. 8 was pierced using a diameter 10 mm punch andusing a diameter 10.5 mm die. FIG. 8 shows the shape of the part seenfrom above. FIG. 8 shows the part 12 and the pierce whole center 13. Thepiercing was performed within 30 minutes after hot shaping. The numberof parts observed was 10. For judgment of the hydrogen embrittlementresistance, the entire circumference of the hole was observed one weekafter piercing to judge the presence of any cracks. The state wasobserved by a loupe or electron microscope. The results of judgment areshown in Tables 3. The judgment criteria are shown below:

Total of number of parts with fine cracks in 10 parts:Very good: 0,

Good: 1,

Fair: less than 5,Poor: 5 or more.A number of parts with fine cracks of less than five was judged aspassing, but of course the smaller the number the better.

As shown in Tables 1-1 to 1-3 and Tables 2-1 to 2-3, if in the scope ofthe present invention, it is learned that it is possible to realizesteel sheet which is sufficiently strengthened by die quenching by hotstamping and is excellent in delayed fracture resistance.

TABLE 1-1 (mass %) Exp. Steel Steel sheet no. type type C Si Mn P S AlTi N Cr Mo 1-1 A HR 0.22 0.005 1.2 0.01 0.002 0.003 0.004 0.003 1 0.22-1 B HR 0.05 0.005 1.5 0.01 0.002 0.003 0.004 0.003 1 0.2 3-1 C HR 0.030.005 1.7 0.01 0.002 0.003 0.004 0.003 1 0.2 4-1 D HR 0.40 0.005 1 0.010.002 0.003 0.004 0.003 1 0.2 1 A CR 0.22 0.005 1.2 0.01 0.002 0.0030.004 0.003 1 0.2 2 B CR 0.05 0.005 1.5 0.01 0.002 0.003 0.004 0.003 10.2 3 C CR 0.03 0.005 1.7 0.01 0.002 0.003 0.004 0.003 1 0.2 4 D CR 0.400.005 1 0.01 0.002 0.003 0.004 0.003 1 0.2 5 A AL 0.22 0.005 1.2 0.010.002 0.003 0.004 0.003 1 0.2 6 B AL 0.05 0.005 1.5 0.01 0.002 0.0030.004 0.003 1 0.2 7 C AL 0.03 0.005 1.7 0.01 0.002 0.003 0.004 0.003 10.2 8 D AL 0.40 0.005 1 0.01 0.002 0.003 0.004 0.003 1 0.2 9 A GI 0.220.005 1.2 0.01 0.002 0.003 0.004 0.003 1 0.2 10 B GI 0.05 0.005 1.5 0.010.002 0.003 0.004 0.003 1 0.2 11 C GI 0.03 0.005 1.7 0.01 0.002 0.0030.004 0.003 1 0.2 12 D GI 0.40 0.005 1 0.01 0.002 0.003 0.004 0.003 10.2 13 A GA 0.22 0.005 1.2 0.01 0.002 0.003 0.004 0.003 1 0.2 14 B GA0.05 0.005 1.5 0.01 0.002 0.003 0.004 0.003 1 0.2 15 C GA 0.03 0.005 1.70.01 0.002 0.003 0.004 0.003 1 0.2 16 D GA 0.40 0.005 1 0.01 0.002 0.0030.004 0.003 1 0.2 17 E GA 0.55 0.005 0.8 0.01 0.002 0.003 0.004 0.003 10.2 18 F CR 0.22 0.05 1.2 0.01 0.002 0.003 0.004 0.003 1 0.2 19 G CR0.22 0.005 3.0 0.01 0.002 0.003 0.004 0.003 0.005 20 H CR 0.22 0.0050.05 0.01 0.002 0.003 0.004 0.003 0.01 21 I CR 0.22 0.005 3.6 0.01 0.0020.003 0.004 0.003 0.01 22 J CR 0.22 0.005 1.2 0.01 0.015 0.003 0.0040.003 1 0.2 23 K CR 0.22 0.005 1.2 0.01 0.024 0.003 0.004 0.003 1 0.2 24L CR 0.22 0.005 1.2 0.025 0.002 0.003 0.004 0.003 1 0.2 25 M CR 0.220.005 1.2 0.035 0.002 0.003 0.004 0.003 1 0.2 26 N CR 0.22 0.005 1.20.01 0.002 0.001 0.004 0.003 1 0.2 27 O CR 0.22 0.005 1.2 0.01 0.0020.04 0.004 0.003 1 0.2 (mass %) Oxide Delayed average Exp. Cr +Martensite fracture particle Crushed no. Mo O B area rate characteristicsize voids Class 1-1 1.2 0.015 G VG G G Inv. ex. 2-1 1.2 0.0162 G VG G GInv. ex. 3-1 1.2 0.0245 × VG G G Comp. ex. 4-1 1.2 0.0104 G G G G Inv.ex. 1 1.2 0.015 G VG G G Inv. ex. 2 1.2 0.0162 G VG G G Inv. ex. 3 1.20.0245 × VG G G Comp. ex. 4 1.2 0.0104 G G G G Inv. ex. 5 1.2 0.015 G VGG G Inv. ex. 6 1.2 0.0162 G VG G G Inv. ex. 7 1.2 0.0245 × VG G G Comp.ex. 8 1.2 0.0104 G G G G Inv. ex. 9 1.2 0.015 G VG G G Inv. ex. 10 1.20.0162 G VG G G Inv. ex. 11 1.2 0.0245 P VG G G Comp. ex. 12 1.2 0.0104G G G G Inv. ex. 13 1.2 0.015 G VG G G Inv. ex. 14 1.2 0.0162 G VG G GInv. ex. 15 1.2 0.0245 P VG G G Comp. ex. 16 1.2 0.0104 G G G G Inv. ex.17 1.2 0.0025 G P P — Comp. ex. 18 1.2 0.0023 G P P — Comp. ex. 19 0.0050.0149 G VG G G Inv. ex. 20 0.01 0.0153 P P — — Comp. ex. 21 0.01 0.0151G P G G Comp. ex. 22 1.2 0.0013 G P P — Comp. ex. 23 1.2 0.0013 G P P —Comp. ex. 24 1.2 0.015 G G G G Inv. ex. 25 1.2 0.015 G F G G Inv. ex. 261.2 0.0161 G VG G G Inv. ex. 27 1.2 0.0022 G P P — Comp. ex.

TABLE 1-2 Ex. Steel Steel sheet no. type type C Si Mn P S Al Ti N Cr Mo28 P CR 0.22 0.005 1.2 0.01 0.002 0.003 0.001 0.003 1 0.2 29 Q CR 0.220.005 1.2 0.01 0.002 0.003 0.04 0.003 1 0.2 30 R CR 0.22 0.005 2 0.010.002 0.003 0.004 0.003 0.005 31 S CR 0.22 0.005 1.8 0.01 0.002 0.0030.004 0.003 0.08 32 T CR 0.22 0.005 1.8 0.01 0.002 0.003 0.004 0.003 0.10 33 U CR 0.22 0.005 1.3 0.01 0.002 0.003 0.004 0.003 0.8 0 34 V CR 0.220.005 0.2 0.01 0.002 0.003 0.004 0.003 0 3 35 W CR 0.22 0.005 1.2 0.010.002 0.003 0.004 0.003 2.5 0 36 X CR 0.22 0.005 1.3 0.01 0.002 0.0030.004 0.003 0.2 37 Y CR 0.15 0.005 1.5 0.01 0.002 0.003 0.004 0.003 0.238 Z CR 0.10 0.005 1.7 0.01 0.002 0.003 0.004 0.003 0.2 39 AA CR 0.030.005 1.8 0.01 0.002 0.003 0.004 0.003 0.2 40 AB CR 0.25 0.005 1.2 0.010.002 0.003 0.004 0.003 0.2 41 AC CR 0.30 0.005 1 0.01 0.002 0.003 0.0040.003 0.2 42 AD CR 0.55 0.005 0.4 0.01 0.002 0.003 0.004 0.003 0.2 43 YAL 0.15 0.005 1.5 0.01 0.002 0.003 0.004 0.003 0.2 44 Z AL 0.10 0.0051.7 0.01 0.002 0.003 0.004 0.003 0.2 45 AA AL 0.03 0.005 1.8 0.01 0.0020.003 0.004 0.003 0.2 46 AB AL 0.25 0.005 1.2 0.01 0.002 0.003 0.0040.003 0.2 47 AC AL 0.30 0.005 1 0.01 0.002 0.003 0.004 0.003 0.2 48 ADAL 0.55 0.005 0.4 0.01 0.002 0.003 0.004 0.003 0.2 49 Y GI 0.15 0.0051.5 0.01 0.002 0.003 0.004 0.003 0.2 50 Z GI 0.10 0.005 1.7 0.01 0.0020.003 0.004 0.003 0.2 51 AA GI 0.03 0.005 1.8 0.01 0.002 0.003 0.0040.003 0.2 52 AB GI 0.25 0.005 1.2 0.01 0.002 0.003 0.004 0.003 0.2 53 ACGI 0.30 0.005 1 0.01 0.002 0.003 0.004 0.003 0.2 54 AD GI 0.55 0.005 0.40.01 0.002 0.003 0.004 0.003 0.2 55 Y GA 0.15 0.005 1.5 0.01 0.002 0.0030.004 0.003 0.2 56 Z GA 0.10 0.005 1.7 0.01 0.002 0.003 0.004 0.003 0.2Oxide Delayed average Ex. Cr + Martensite fracture particle Crushed no.Mo O B area rate characteristic size voids Class 28 1.2 0.03 G VG G GInv. ex. 29 1.2 0.0013 G P P — Comp. ex. 30 0.005 0.0149 F VG G G Inv.ex. 31 0.08 0.0153 F VG G G Inv. ex. 32 0.1 0.0148 F VG G G Inv. ex. 330.8 0.0145 F VG G G Inv. ex. 34 3 0.0154 G P G G Comp. ex. 35 2.5 0.015G P G G Comp. ex. 36 0.2 0.0163 0.0048 G VG G G Inv. ex. 37 0.2 0.01830.0052 G VG G G Inv. ex. 38 0.2 0.0193 0.0048 G VG G G Inv. ex. 39 0.20.0233 0.0048 P VG G G Comp. ex. 40 0.2 0.0134 0.0045 G VG G G Inv. ex.41 0.2 0.0121 0.0054 G VG G G Inv. ex. 42 0.2 0.0025 0.0043 G P P —Comp. ex. 43 0.2 0.0183 0.0052 G VG G G Inv. ex. 44 0.2 0.0193 0.0048 GVG G G Inv. ex. 45 0.2 0.0233 0.0048 P VG G G Comp. ex. 46 0.2 0.01340.0045 G VG G G Inv. ex. 47 0.2 0.0121 0.0054 G VG G G Inv. ex. 48 0.20.0025 0.0043 G P P — Comp. ex. 49 0.2 0.0183 0.0052 G VG G G Inv. ex.50 0.2 0.0193 0.0048 G VG G G Inv. ex. 51 0.2 0.0233 0.0048 P VG G GComp. ex. 52 0.2 0.0134 0.0045 G VG G G Inv. ex. 53 0.2 0.0121 0.0054 GVG G G Inv. ex. 54 0.2 0.0025 0.0043 G P P — Comp. ex. 55 0.2 0.01830.0052 G VG G G Inv. ex. 56 0.2 0.0193 0.0048 G VG G G Inv. ex.

TABLE 1-3 Oxide Steel Delayed average Ex. Steel sheet Cr + Martensitefracture particle Crushed no. type type C Si Mn P S Al Ti N Cr Mo Mo O Barea rate characteristic size voids Class 57 AA GA 0.03 0.005 1.8 0.010.002 0.003 0.004 0.003 0.2 0.2 0.0233 0.0048 P VG G G Comp. ex. 58 ABGA 0.25 0.005 1.2 0.01 0.002 0.003 0.004 0.003 0.2 0.2 0.0134 0.0045 GVG G G Inv. ex. 59 AC GA 0.30 0.005 1 0.01 0.002 0.003 0.004 0.003 0.20.2 0.0121 0.0054 G VG G G Inv. ex. 60 AD GA 0.55 0.005 0.4 0.01 0.0020.003 0.004 0.003 0.2 0.2 0.0025 0.0043 G P P — Comp. ex. 61 AE CR 0.220.001 1.3 0.01 0.002 0.003 0.004 0.003 0.2 0.2 0.0173 0.0044 G VG G GInv. ex. 62 AF CR 0.22 0.007 1.3 0.01 0.002 0.003 0.004 0.003 0.2 0.20.0103 0.0048 G G G G Inv. ex. 63 AG CR 0.22 0.014 1.3 0.01 0.002 0.0030.004 0.003 0.2 0.2 0.003 0.0049 G F G G Inv. ex. 63-1 AG2 CR 0.22 0.021.3 0.01 0.002 0.003 0.004 0.003 0.2 0.2 0.003 0.0049 G F G G Inv. ex.64 AH CR 0.22 0.023 1.3 0.01 0.002 0.003 0.004 0.003 0.2 0.2 0.00130.0049 G P P — Comp. ex. 65 AI CR 0.22 0.005 0.03 0.01 0.002 0.003 0.0040.003 0.2 0.2 0.0144 0.0053 P P — — Comp. ex. 65-1 AI2 CR 0.22 0.005 0.10.01 0.002 0.003 0.004 0.003 0.2 0.2 0.0144 0.0053 G F G G Inv. ex. 66AJ CR 0.22 0.005 3.3 0.01 0.002 0.003 0.004 0.003 0.2 0.2 0.0155 0.0048G P G G Comp. ex. 67 AK CR 0.22 0.005 1.3 0.01 0.002 0.003 0.004 0.0030.2 0.2 0.0157 0.0053 G VG G G Inv. ex. 68 AL CR 0.22 0.005 1.3 0.010.013 0.003 0.004 0.003 0.2 0.2 0.0148 0.0055 G G G G Inv. ex. 69 AM CR0.22 0.005 1.3 0.01 0.032 0.003 0.004 0.003 0.2 0.2 0.0153 0.0054 G F GG Inv. ex. 70 AN CR 0.22 0.005 1.3 0.025 0.002 0.003 0.004 0.003 0.2 0.20.0163 0.0048 G G G G Inv. ex. 71 AO CR 0.22 0.005 1.3 0.035 0.002 0.0030.004 0.003 0.2 0.2 0.0163 0.0048 G F G G Inv. ex. 72 AP CR 0.22 0.0051.3 0.01 0.002 0.0002 0.004 0.003 0.2 0.2 0.024 0.0053 G G G G Inv. ex.73 AQ CR 0.22 0.005 1.3 0.01 0.002 0.0012 0.004 0.003 0.2 0.2 0.01830.0054 G VG G G Inv. ex. 74 AR CR 0.22 0.005 1.3 0.01 0.002 0.005 0.0040.003 0.2 0.2 0.0102 0.0053 G G G G Inv. ex. 75 AS CR 0.22 0.005 1.30.01 0.002 0.0073 0.004 0.003 0.2 0.2 0.0018 0.0047 G P P — Comp. ex. 76AT CR 0.22 0.005 1.3 0.01 0.002 0.003 0.0005 0.003 0.2 0.2 0.0173 0.0045G VG G G Inv. ex. 77 AU CR 0.22 0.005 1.3 0.01 0.002 0.003 0.001 0.0030.2 0.2 0.0166 0.0045 G VG G G Inv. ex. 78 AV CR 0.22 0.005 1.3 0.010.002 0.003 0.01 0.003 0.2 0.2 0.0107 0.0054 G G G G Inv. ex. 79 AW CR0.22 0.005 1.3 0.01 0.002 0.003 0.023 0.003 0.2 0.2 0.0008 0.0055 G P P— Comp. ex. 80 AX CR 0.22 0.005 1.3 0.01 0.002 0.003 0.004 0.003 0.0080.07 0.078 0.0145 0.0058 G VG G G Inv. ex. 81 AY CR 0.22 0.005 1.3 0.010.002 0.003 0.004 0.003 0.02 0.1 0.12 0.0156 0.0049 G VG G G Inv. ex. 82AZ CR 0.22 0.005 0.5 0.01 0.002 0.003 0.004 0.003 1.2 1.2 0.0161 0.0053G VG G G Inv. ex. 83 BA CR 0.22 0.005 0.3 0.01 0.002 0.003 0.004 0.0030.7 0.3 1 0.0146 0.0055 G VG G G Inv. ex. 84 BB CR 0.22 0.005 0.5 0.010.002 0.003 0.004 0.003 0.02 2.2 2.22 0.0153 0.005 G P G G Comp. ex. 85BC CR 0.22 0.005 0.5 0.01 0.002 0.003 0.004 0.003 1.7 0.3 2 0.01530.0048 G F G G Inv. ex. 86 BD CR 0.22 0.005 1.3 0.01 0.002 0.003 0.0040.003 0.8 0.4 1.2 0.0155 0.0005 G VG G G Inv. ex. 87 BE CR 0.22 0.0051.3 0.01 0.002 0.003 0.004 0.003 0.8 0.4 1.2 0.0155 0.001 G VG G G Inv.ex. 88 BF CR 0.22 0.005 1.3 0.01 0.002 0.003 0.004 0.003 0.9 0.2 1.10.0143 0.0024 G VG G G Inv. ex. 89 BG CR 0.22 0.005 1.3 0.01 0.002 0.0030.004 0.003 0.2 0.2 0.0134 0.0073 G VG G G Inv. ex. 90 BH CR 0.22 0.0051.3 0.01 0.002 0.003 0.004 0.003 0.2 0.2 0.0143 0.0134 G F G G Inv. ex.

TABLE 2-1 (mass %) Oxide Base Steel Delayed average Steel steel sheetNb + V + Ni + Martensite fracture particle Crushed Ex. no. type typetype Nb V Co W Ni Cu Co + W Cu area rate characteristic size voids Class91 BI A CR 0.02 0.01 0.03 G VG G G Inv. ex. 92 BJ A CR 0.01 0.032 0.04 GVG G G Inv. ex. 93 BK A CR 0.5 0.50 G VG G G Inv. ex. 94 BL A CR 1 1.00G VG G G Inv. ex. 95 BM A CR 0.5 0.04 0.54 G VG G G Inv. ex. 96 BN A CR1.4 0.8 2.20 G F G G Inv. ex. 97 BO A CR 1 1.5 2.50 G F G G Inv. ex. 98BP A CR 0.008 0.008 G VG G G Inv. ex. 99 BQ A CR 0.03 0.030 G VG G GInv. ex. 100 BR A CR 0.08 0.080 G VG G G Inv. ex. 101 BS A CR 0.05 0.050G VG G G Inv. ex. 102 BT A CR 0.5 0.500 G VG G G Inv. ex. 103 BU A CR0.8 0.800 G VG G G Inv. ex. 104 BV A CR 0.03 0.030 G VG G G Inv. ex. 105BW A CR 0.02 0.020 G VG G G Inv. ex. 106 BX A CR 0.03 0.2 0.230 G VG G GInv. ex. 107 BY A CR 0.05 0.3 0.350 G VG G G Inv. ex. 108 BZ A CR 0.040.03 0.070 G VG G G Inv. ex. 109 CA A CR 0.08 0.2 0.280 G VG G G Inv.ex. 110 CB A CR 0.08 0.5 0.1 0.8 1.480 G F G G Inv. ex. 111 CC A CR 0.040.3 0.01 0.03 0.340 0.04 G VG G G Inv. ex. 112 CD A CR 0.04 0.3 1 0.030.340 1.03 G VG G G Inv. ex. 113 CE A CR 0.04 0.3 1.3 0.5 0.340 1.800 GVG G G Inv. ex. 114 CF A CR 0.04 0.3 0.1 1.3 0.5 0.440 1.800 G VG G GInv. ex. 115 CG A CR 0.1 0.3 0.1 1.3 0.7 0.500 2.000 G VG G G Inv. ex.116 CH A CR 0.55 0.3 0.1 0.05 1.3 0.5 1.000 1.800 G VG G G Inv. ex. 117CI X CR 0.02 0.01 0.03 G VG G G Inv. ex. 118 CJ X CR 0.01 0.032 0.04 GVG G G Inv. ex. 119 CK X CR 0.5 0.50 G VG G G Inv. ex. 120 CL X CR 11.00 G VG G G Inv. ex. 121 CM X CR 0.5 0.04 0.54 G VG G G Inv. ex. 122CN X CR 1.4 0.8 2.20 G F G G Inv. ex.

TABLE 2-2 (mass %) Oxide Base Steel Delayed average Steel steel sheetNb + V + Ni + Martensite fracture particle Crushed Ex. no. type typetype Nb V Co W Ni Cu Co + W Cu area rate characteristic size voids Class123 CO X CR 1 1.5 2.50 G F G G Inv. ex. 124 CP X CR 0.005 0.005 G VG G GInv. ex. 125 CQ X CR 0.032 0.032 G VG G G Inv. ex. 126 CR X CR 0.0810.081 G VG G G Inv. ex. 127 CS X CR 0.053 0.053 G VG G G Inv. ex. 128 CTX CR 0.48 0.480 G VG G G Inv. ex. 129 CU X CR 0.79 0.790 G VG G G Inv.ex. 130 CV X CR 0.03 0.030 G VG G G Inv. ex. 131 CW X CR 0.02 0.020 G VGG G Inv. ex. 132 CX X CR 0.03 0.2 0.230 G VG G G Inv. ex. 133 CY X CR0.048 0.3 0.348 G VG G G Inv. ex. 134 CZ X CR 0.04 0.03 0.070 G VG G GInv. ex. 135 DA X CR 0.08 0.2 0.280 G VG G G Inv. ex. 136 DB X CR 0.090.5 0.1 0.8 1.490 G F G G Inv. ex. 137 DC X CR 0.05 0.3 0.01 0.03 0.3500.04 G VG G G Inv. ex. 138 DD X CR 0.05 0.3 1 0.03 0.350 1.03 G VG G GInv. ex. 139 DE X CR 0.05 0.3 1.3 0.5 0.350 1.800 G VG G G Inv. ex. 140DF X CR 0.05 0.3 0.1 1.3 0.5 0.450 1.800 G VG G G Inv. ex. 141 DG X CR0.15 0.3 0.1 1.3 0.7 0.550 2.000 G VG G G Inv. ex. 142 DH X CR 0.55 0.30.1 0.05 1.3 0.5 1.000 1.800 G VG G G Inv. ex. 143 DC X AL 0.05 0.3 0.010.03 0.350 0.04 G VG G G Inv. ex. 144 DD X AL 0.05 0.3 1 0.03 0.350 1.03G VG G G Inv. ex. 145 DE X AL 0.05 0.3 1.3 0.5 0.350 1.800 G VG G G Inv.ex. 146 DF X AL 0.05 0.3 0.1 1.3 0.5 0.450 1.800 G VG G G Inv. ex. 147DG X AL 0.15 0.3 0.1 1.3 0.7 0.550 2.000 G VG G G Inv. ex. 148 DH X AL0.55 0.3 0.1 0.05 1.3 0.5 1.000 1.800 G VG G G Inv. ex. 149 DC X GI 0.050.3 0.01 0.03 0.350 0.04 G VG G G Inv. ex. 150 DD X GI 0.05 0.3 1 0.030.350 1.03 G VG G G Inv. ex. 151 DE X GI 0.05 0.3 1.3 0.5 0.350 1.800 GVG G G Inv. ex. 152 DF X GI 0.05 0.3 0.1 1.3 0.5 0.450 1.800 G VG G GInv. ex. 153 DG X GI 0.15 0.3 0.1 1.3 0.7 0.550 2.000 G VG G G Inv. ex.154 DH X GI 0.55 0.3 0.1 0.05 1.3 0.5 1.000 1.800 G VG G G Inv. ex. 155DC X GA 0.05 0.3 0.01 0.03 0.350 0.04 G VG G G Inv. ex.

TABLE 2-3 (mass %) Oxide Base Steel Delayed average Steel steel sheetNb + V + Ni + Martensite fracture particle Crushed Ex. no. type typetype Nb V Co W Ni Cu Co + W Cu area rate characteristic size voids Class156 DD X GA 0.05 0.3 1 0.03 0.350 1.03 G VG G G Inv. ex. 157 DE X GA0.05 0.3 1.3 0.5 0.350 1.800 G VG G G Inv. ex. 158 DF X GA 0.05 0.3 0.11.3 0.5 0.450 1.800 G VG G G Inv. ex. 159 DG X GA 0.15 0.3 0.1 1.3 0.70.550 2.000 G VG G G Inv. ex. 160 DH X GA 0.55 0.3 0.1 0.05 1.3 0.51.000 1.800 G VG G G Inv. ex. 161 DI AC CR 0.02 0.01 0.03 G VG G G Inv.ex. 162 DJ AC CR 0.01 0.032 0.04 G VG G G Inv. ex. 163 DK AC CR 0.5 0.50G VG G G Inv. ex. 164 DL AC CR 1 1.00 G VG G G Inv. ex. 165 DM AC CR 0.50.04 0.54 G VG G G Inv. ex. 166 DN AC CR 1.4 0.8 2.20 G F G G Inv. ex.167 DO AC CR 1 1.5 2.50 G F G G Inv. ex. 168 DP AC CR 0.005 0.005 G VG GG Inv. ex. 169 DQ AC CR 0.032 0.032 G VG G G Inv. ex. 170 DR AC CR 0.0810.081 G VG G G Inv. ex. 171 DS AC CR 0.053 0.053 G VG G G Inv. ex. 172DT AC CR 0.48 0.480 G VG G G Inv. ex. 173 DU AC CR 0.79 0.790 G VG G GInv. ex. 174 DV AC CR 0.03 0.030 G VG G G Inv. ex. 175 DW AC CR 0.020.020 G VG G G Inv. ex. 176 DX AC CR 0.03 0.2 0.230 G VG G G Inv. ex.177 DY AC CR 0.048 0.3 0.348 G VG G G Inv. ex. 178 DZ AC CR 0.04 0.030.070 G VG G G Inv. ex. 179 EA AC CR 0.08 0.2 0.280 G VG G G Inv. ex.180 EB AC CR 0.09 0.5 0.1 0.8 1.490 G F G G Inv. ex. 181 EC AC CR 0.050.3 0.01 0.03 0.350 0.04 G VG G G Inv. ex. 182 ED AC CR 0.05 0.3 1 0.030.350 1.03 G VG G G Inv. ex. 183 EF AC CR 0.05 0.3 1.3 0.5 0.350 1.800 GVG G G Inv. ex. 184 EG AC CR 0.05 0.3 0.1 1.3 0.5 0.450 1.800 G VG G GInv. ex. 185 EH AC CR 0.15 0.3 0.1 1.3 0.7 0.550 2.000 G VG G G Inv. ex.186 EI AC CR 0.55 0.3 0.1 0.05 1.3 0.5 1.000 1.800 G VG G G Inv. ex.

Example 2

The Steel Types A, X, and AC which are shown in Tables 1-1 and 1-2 wereused to study the rolling conditions. These slabs were heated to 1050 to1350° C. then hot rolled by a finish temperature of 800 to 900° C. and acoiling temperature of 450 to 680° C. to obtain hot rolled steel sheets.The slabs, rough rolled sheets, the thickness and rough rolling rate ofthe hot rolled sheets, and the final rolling rate are shown in Tables3-1 and 3-2. After that, part of the hot rolled steel sheets werepickled, then cold rolled. The cold rolled sheet thickness and coldrolling rate are shown in Tables 3-1 and 3-2. After that, part of thesteel sheets was continuously annealed (annealing temperature 720 to830° C.). Further, parts of the steel sheets were hot dip galvanized(basis weight: one side 30 to 90 g/m²), hot dip galvannealed (basisweight: one side 30 to 90 g/m²), and hot dip aluminum coated (basisweight: one side 30 to 100 g/m²) on a continuous hot dipping line. Thesteel sheet types are shown in Tables 3. The types of steel sheets areshown below:

HR: hot rolled steel sheet, CR: cold rolled steel sheet (annealedmaterial), AL: hot dip aluminum coated steel sheet, GI: hot dipgalvanized steel sheet, and GA: hot dip galvannealed steel sheet.

The average particle size of the Fe—Mn composite oxides in a producedsteel sheet and the presence of crushed voids were determined bypolishing a cross-section of the steel sheet, then observing it by anoptical microscope or scan type electron microscope or by a transmissiontype electron microscope after preparing the sample into a thin film.The results are shown in Tables 3-1 to 3-2. The judgment criteria areshown below:

Average particle size of composite oxides:Good: average diameter 0.1 to 15 μm,Poor: average diameter less than 0.1 μm or over 15 μmCrushed voids around composite oxides:Good: average size of voids 0.1 μm or morePoor: average size of voids less than 0.1 μm

After that, these cold rolled steel sheets were heated by a heatingfurnace to over the Ac3 point, that is, the 880 to 950° C. austeniteregion, then were hot worked. For the atmosphere of the heating furnace,combustion exhaust gas was used. The hydrogen concentration in theatmosphere was 2%, while the dew-point was 20° C.

The cross-section of the shape of the die set which is used in theexamples is shown in FIG. 4. FIG. 4 shows the shapes of the die 9 andpunch 10. The shape of the punch as seen from above is shown in FIG. 5.FIG. 5 shows the punch 10. The shape of the die as seen from below isshown in FIG. 6. FIG. 6 shows the die 9. In the die set, the shape ofthe die is determined based on the punch with a clearance of the sheetthickness of 1.6 mm. The blank size was made 1.6 mm thickness×300 mm×500mm. The shaping conditions were made a punch speed of 10 mm/s, apressing force of 200 tons, and a holding time at bottom dead center of5 seconds. A schematic view of the hot pressed part is shown in FIG. 7.

The quenching characteristic of the steel sheet was evaluated bypolishing the cross-section, corroding it by Nital, then observing themicrostructure by an optical microscope and determining the area rate ofmartensite. The results of judgment are shown in Tables 3-1 and 3-2. Thejudgment criteria are shown below:

Good: martensite area rate 90% or more,Fair: martensite area rate 80% or more, andPoor: martensite area rate less than 80%.

The delayed fracture resistance was evaluated by imparting stress bypiercing. The pierce hole position 13 at the center of the test part 12which is shown in FIG. 8 was pierced using a diameter 10 mm punch andusing a die of a diameter giving a clearance 15%±2. FIG. 8 shows theshape of the part seen from above. FIG. 8 shows the part 12 and thepierce hole center 13. The piercing was performed within 30 minutesafter hot shaping. The number of parts observed was 10. For judgment ofthe hydrogen embrittlement resistance, the entire circumference of thehole was observed one week after piercing to judge the presence of anycracks. The state was observed by a loupe or electron microscope. Theresults of judgment are shown in Tables 3-1 and 3-2. The judgmentcriteria are shown below:

Total of number of parts with fine cracks in 10 parts:Very good: 0,

Good: 1,

Fair: less than 5, andPoor: 5 or more.

As shown in Tables 3-1 and 3-2, it is learned that if in the scope ofthe method of production which is recommended by the present invention,steel sheet can be realized which is sufficiently strengthened by diequenching by hot stamping and which is more excellent in delayedfracture resistance.

TABLE 3-1 Rough Hot Cold Slab rolling rolling rolling Rough Final ColdOxide Steel thick- thick- thick- thick- rolling rolling rolling Delayedaverage Steel sheet ness ness ness ness rate rate rate Martensitefracture particle Crushed Ex. no. type Rolling type (mm) (mm) (mm) (mm)(%) (%) (%) area rate rate size void Class 187 A HR N 250 20 6.5 92 67.5G P P P Comp. ex. 187-1 A HR N 250 20 6 92 70.0 G G G G Inv. ex. 188 AHR N 250 30 6.5 88 78.3 G F G G Inv. ex. 189 A HR N 250 40 6.5 84 83.8 GG G G Inv. ex. 190 A HR N 100 40 3 60 92.5 G P P P Comp. ex. 191 A HR N150 40 3 73.333 92.5 G F G G Inv. ex. 192 A HR AL 150 40 3 73.333 92.5 GF G G Inv. ex. 193 A HR GI 150 40 3 73.333 92.5 G F G G Inv. ex. 194 AHR GA 150 40 3 73.333 92.5 G F G G Inv. ex. 195 A HR N 200 40 3 80 92.5G G G G Inv. ex. 196 A HR N 250 40 3 84 92.5 G G G G Inv. ex. 197 A CR N250 40 1.5 1.2 84 96.3 20.0 G P P G Comp. ex. 198 A CR N 250 40 1.9 1.284 95.3 36.8 G F G G Inv. ex. 199 A CR N 250 40 2.5 1.2 84 93.8 52.0 G GG G Inv. ex. 200 A CR A 250 40 2.5 1.2 84 93.8 52.0 G G G G Inv. ex. 201A CR AL 250 40 2.5 1.2 84 93.8 52.0 G G G G Inv. ex. 202 A CR GI 250 402.5 1.2 84 93.8 52.0 G G G G Inv. ex. 203 A CR GA 250 40 2.5 1.2 84 93.852.0 G G G G Inv. ex. 204 A CR N 250 40 3 1.2 84 92.5 60.0 G VG G G Inv.ex. 205 A CR N 250 40 4 1.2 84 90.0 70.0 G VG G G Inv. ex. 206 A CR N250 40 5 1.2 84 87.5 76.0 G VG G G Inv. ex. 207 X HR N 250 20 6.5 9267.5 G P P P Comp. ex. 207-1 X HR N 250 20 6 92 70.0 G G G G Inv. ex.208 X HR N 250 30 6.5 88 78.3 G F G G Inv. ex. 209 X HR N 250 40 6.5 8483.8 G G G G Inv. ex. 210 X HR N 100 40 3 60 92.5 G P P P Comp. ex. 211X HR N 150 40 3 73.333 92.5 G F G G Inv. ex. 212 X HR N 200 40 3 80 92.5G G G G Inv. ex. 213 X HR N 250 40 3 84 92.5 G G G G Inv. ex. 214 X HRAL 250 40 3 84 92.5 G G G G Inv. ex. 215 X HR GI 250 40 3 84 92.5 G G GG Inv. ex.

TABLE 3-2 Rough Hot Cold Slab rolling rolling rolling Rough Final ColdOxide Steel thick- thick- thick- thick- rolling rolling rolling Delayedaverage Steel sheet ness ness ness ness rate rate rate Martensitefracture particle Crushed Ex. no. type Rolling type (mm) (mm) (mm) (mm)(%) (%) (%) area rate rate size void Class 216 X CR GA 250 40 3 84 92.5G G G G Inv. ex. 217 X CR N 250 40 1.5 1.2 84 96.3 20.0 G P P G Comp.ex. 218 X CR N 250 40 1.9 1.2 84 95.3 36.8 G F G G Inv. ex. 219 X CR N250 40 2.5 1.2 84 93.8 52.0 G G G G Inv. ex. 220 X CR N 250 40 3 1.2 8492.5 60.0 G VG G G Inv. ex. 221 X CR N 250 40 4 1.2 84 90.0 70.0 G VG GG Inv. ex. 222 X CR A 250 40 4 1.2 84 90.0 70.0 G G G G Inv. ex. 223 XCR AL 250 40 4 1.2 84 90.0 70.0 G G G G Inv. ex. 224 X CR GI 250 40 41.2 84 90.0 70.0 G G G G Inv. ex. 225 X CR GA 250 40 4 1.2 84 90.0 70.0G G G G Inv. ex. 226 X CR N 250 40 5 1.2 84 87.5 76.0 G VG G G Inv. ex.227 AC HR N 250 20 6.5 92 67.5 G P P P Comp. ex. 228 AC HR N 250 30 6.588 78.3 G F G G Inv. ex. 229 AC HR N 250 40 6.5 84 83.8 G G G G Inv. ex.230 AC HR N 100 40 3 60 92.5 G P P P Comp. ex. 231 AC HR N 150 40 373.333 92.5 G F G G Inv. ex. 232 AC HR N 200 40 3 80 92.5 G G G G Inv.ex. 233 AC HR AL 200 40 3 80 92.5 G G G G Inv. ex. 234 AC HR GI 200 40 380 92.5 G G G G Inv. ex. 235 AC HR GA 200 40 3 80 92.5 G G G G Inv. ex.236 AC HR N 250 40 3 84 92.5 G G G G Inv. ex. 237 AC CR N 250 40 1.5 1.284 96.3 20.0 G P P G Comp. ex. 238 AC CR N 250 40 1.9 1.2 84 95.3 36.8 GF G G Inv. ex. 239 AC CR A 250 40 1.9 1.2 84 95.3 36.8 G F G G Inv. ex.240 AC CR AL 250 40 1.9 1.2 84 95.3 36.8 G F G G Inv. ex. 241 AC CR GI250 40 1.9 1.2 84 95.3 36.8 G F G G Inv. ex. 242 AC CR GA 250 40 1.9 1.284 95.3 36.8 G F G G Inv. ex. 243 AC CR N 250 40 2.5 1.2 84 93.8 52.0 GG G G Inv. ex. 244 AC CR N 250 40 3 1.2 84 92.5 60.0 G VG G G Inv. ex.245 AC CR N 250 40 4 1.2 84 90.0 70.0 G VG G G Inv. ex. 246 AC CR N 25040 5 1.2 84 87.5 76.0 G VG G G Inv. ex.

INDUSTRIAL APPLICABILITY

The present invention can be used as a steel material for hot stampinguse. Regarding its field of use, this can be utilized in a broad rangeof industrial fields such as auto parts, home electrical appliances,machinery, etc.

REFERENCE SIGNS LIST

-   -   1 coarse composite oxides    -   1-1, 1-2 oxides    -   2 hot rough rolling    -   3 stretched composite oxides    -   3-1, 3-2 stretched oxides    -   4 hot final rolling    -   5 crushed void (hydrogen trapping ability)    -   5-1 and 5-2 crushed oxides    -   6 coarse oxides    -   6′ fine oxides    -   7 stretched oxides    -   8 crushed oxides    -   9 die    -   10 punch    -   11 hot stamped part    -   12 test part    -   13 pierced hole position

1. A method of production of steel sheet for hot stamping use comprisinghot rolling a slab of chemical ingredients in which rough rolling theslab by a rolling rate of 70% or more and final rolling the slab by arolling rate of 70% or more, wherein the chemical ingredients contain,by mass %, C: 0.05 to 0.40%, Si: 0.001 to 0.02%, Mn: 0.1 to 3%, Al:0.0002 to 0.005%, Ti: 0.0005 to 0.01%, O: 0.003 to 0.03%, one or more ofCr and Mo in a total of 0.005 to 2%, and a balance of Fe and unavoidableimpurities.
 2. The method of production of steel sheet for hot stampinguse as set forth in claim 1, wherein the chemical ingredients furthercontain, by mass %, an ingredient(s) which is included in one or moregroups among the three groups of (a) to (c): (a) B: 0.0005 to 0.01%; (b)one or more of Nb, V, W, and Co in a total of 0.005 to 1%; and (c) oneor more of Ni and Cu in a total of 0.005 to 2%.
 3. The method ofproduction of steel sheet for hot stamping use as set forth in claim 1,further comprising pickling the hot rolled steel sheet which wasobtained by hot rolling and cold rolling the steel sheet by a rollingrate of 30% or more.
 4. The method of production of steel sheet for hotstamping use as set forth in claim 2, further comprising pickling thehot rolled steel sheet which was obtained by hot rolling and coldrolling the steel sheet by a rolling rate of 30% or more.
 5. The methodof production of steel sheet for hot stamping use as set forth in claim3, further comprising annealing the cold rolled steel sheet which wasobtained by cold rolling.
 6. The method of production of steel sheet forhot stamping use as set forth in claim 4, further comprising annealingthe cold rolled steel sheet which was obtained by cold rolling.
 7. Amethod of production of a high strength part using a steel sheet for hotstamping use comprising heating the steel sheet to a temperature ofaustenite region of the Ac₃ or higher, then starting to form the steelsheet by a die, and cooling the steel sheet in the die after forming toquench, wherein the steel sheet is comprised of chemical ingredientswhich contain, by mass %, C: 0.05 to 0.40%, Si: 0.001 to 0.02%, Mn: 0.1to 3%, Al: 0.0002 to 0.005%, Ti: 0.0005 to 0.01%, O: 0.003 to 0.03%, oneor more of Cr and Mo in a total of 0.005 to 2%, and a balance of Fe andunavoidable impurities, and wherein the steel sheet contains averagediameter 0.1 to 15 μm Fe—Mn-based composite oxide particles dispersed inthe steel sheet.
 8. The method of production of a high strength partusing a steel sheet for hot stamping use as set forth in claim 7,wherein the chemical ingredients further contain, by mass %, aningredient(s) which is included in one or more groups among the threegroups of (a) to (c): (a) B: 0.0005 to 0.01%; (b) one or more of Nb, V,W, and Co in a total of 0.005 to 1%; and (c) one or more of Ni and Cu ina total of 0.005 to 2%.